Advance Structural Frames Coursework Activity

Unit 1: Advance Structural Frames Coursework Activity

1(a) The structural frame is undeniably a vital element of any building and it is normally can be formed either in steel or concrete. Nowadays the most commonly choices of structural frame used in the building construction can be categories as below.

  • Insitu reinforcement concrete frame construction
  • Steel frame construction
  • Precast concrete frame construction

The structural frame design idea will be influence and consider in terms of the building functionality, building form, engineering alternatives, market conditions and experience, as well as the building configuration. Besides it is also play significant factors for the successful of any particular building, in term of short and long term.

Therefore in this early stage of propose new office development, which is consists of a ground floor and 9 upper floor. The selection of structural frames have been study carefully as tabulation below (Figure A ) in order to achieve the overall vision of our client to build one of high tech, using the most up to date materials and method of construction building.

Item  Description               Insitu Reinforcement          Steel Frame                       Precast Concrete 
                                Concrete Construction        Construction                       Frame Construction

1   Suitable selection of   Stable                      Unstable, subject to market       Unstable, subject to 
    materials and labour                                 demand and condition             market demand and condition
2   Cost                     Low                         High                              Low
3   Speed of construction    Relatively slow in term of  Can be erected at a faster pace.  Can be erected at a faster pace.
    and lead time            installation speed.         But involved longer lead time     But involved longer lead time
4   Ability to standardise   Yes                         No                                Tend to be standardise
5   Fire resistance          Inbuilt and not requires    Not inbuilt and requires          Inbuilt and not requires
                             additional fire protection  additional fire protection works  additional fire protection 
                             works.                                                        works.
6   Size and nature of site  Unlimited                   Limited                           Limited
            

Figure A

From the above evaluation, we recommended that the most suitable frame material and type of frame materials for this prestigious project should be the 'Insitu reinforcement concrete' based on the advantages as mentioned below

Suitable selection of materials and labour

Insitu reinforcement concrete and structural steel are popular and dominates in the framing market of multi storey buildings. Through the above evaluation we found that the insitu reinforcement concrete frame's materials and labour is pretty much stable then steel and precast concrete frame. Therefore, we recommend using insitu reinforcement concrete frame approach in this development.

Cost

Cost is one of the crucial factors need to be considered in the selection of structural framing options and this costs of framed structures can be affected by the market condition. Through our experience and market analysis, we found that the insitu reinforcement concrete frame is much cost efficiency compared to the others type of structural frame. This is due to steel is particularly volatile and always influence by the exchange rates due to international competitions. Adversely, cement price is more stable and according to 'BCA (2007)', the statistic claims that the UK average cement price is stable over the last 10 years and it is risen below the inflation rate. Cement is one of the main component in the concrete mixture and thus cause the concrete price generally will be more cheaper. Therefore it is wisely no to propose steel frame as its price tends to fluctuate more frequently than that of concrete.

Speed of construction

In principle, the insitu reinforcement concrete frame has a disadvantage in term of construction speed it is relatively slow due to the time consumed for curing. However, lead time for steel frame actually is higher than insitu reinforcement concrete frame because one of the reasons is due to the steel frame need to pre-fabricate in factory and it is requires a number of fabrication processes.

Lead time can be defined as the actual time counted from putting in an order (by the builder) until to actual construction of a particular element on site. According to the "lead time figure published in the Chartered Quantity Surveyor (1992), show that insitu reinforcement concrete frame's lead times is 2-8 weeks and it is less than the steel frame's lead times; with 8-14 weeks". Hence, the insitu reinforcement frame construction's "total construction time" would be shorter if compared to steel frame. Although steel frame construction has an advantage in term of faster structural erection time but in our opinion the shorter 'total construction time' produced by insitu reinforcement frame construction is more crucial. Therefore, the insitu reinforcement concrete frame construction is more suitable and is recommended in this project.

Ability to standardise

The insitu reinforcement concrete frame is more flexible and tolerant in any alteration during the construction process. Any subsequence alteration is straightforward and it would not much affect the following construction sequence, process, cost and importantly greater delay would not happen. But, both the steel and precast concrete frame has disadvantages in the ability to standardise. This is due to both are factory prefabricated products and later only deliver to site for installation. Therefore any subsequence alteration in steel frame or precast concrete frame construction either in design or construction sequence will cause an impact in the factory production line as well as to the subsequent following construction planning process.

Fire resistance

The insitu reinforcement concrete frame has inherent fire resistance advantage compared to steel frame which fire resistance factor is not inbuilt. Therefore steel need additional fire protection work and this directly will involve additional construction time and cost.

Size and nature of site

As mentioned above both of the steel and precast concrete frame are prefabricated in the factory. Therefore, it has a disadvantage and limitation in producing huge structural frame during the prefabrication process and installation process at site. Similarly the delivery process of prefabricated huge frame will also tough and massive and not practical. But, this does not happen to the insitu reinforcement concrete frame where the huge structural elements can be adjusted and produced on site by constructing it in small part each time without affect by the factor of size and nature of site.

Finally we would like to propose that this 10 storey building to be constructed by using insitu reinforcement concrete frame construction due to the above mentioned advantages. In addition, this insitu reinforcement concrete frame is more useful in accomplishing 'green process 'compared to steel frame. Adversely, we also do not negligence on the disadvantages points of the insitu reinforcement concrete frame such as quality control, massive construction process and etc. However, this insitu reinforcement concrete's existent disadvantages factors can be reduced to an acceptable level by adopting a proper site management system and well planned construction process.

REFERENCES:

1. Construction Technology 5, Heriot-Watt University

2. Comparison Of Reinforced In-Situ Concrete And Structural Steel In Multi-Storey Building Framework Construction, RIAD QUADERY (ICE Membership Number: 64405090)

3. BCA (2007

4. Chartered Quantity Surveyor (1992)

Unit 1 (c)

In order to increase the building height to 30 storeys, the previous proposed structure need to consider and cope efficiently to the gravity loads and resists significant lateral loads or "sway" force cause by wind, while at the same time, not presenting excessive self weight loads on the foundation system. Therefore, some alteration in terms of frame will need to take into consideration for achieving the tall building design safety factor.

According to "Chew, Y L M., Construction Technology for Tall Buildings (2nd Edition), the amount of materials needed in a tall building to resist gravity loads is almost linear with its height, however the amount of material needed to withstand lateral forces increases with the square of the wind speed." The Figure 2.1 below is an illustration of the lateral forces imposed by wind increase exponentially with the building height.

A) Introduction of "Shear Truss - Shear wall Structure"

Shear walls comprise the vertical elements in the lateral force resisting system (LFRS) for many structures. Shear wall within a conventional concrete frame system, actually it is function as a deep, thin vertical cantilevered beams members where it is robustly connected from the roof level onto the foundations level. While at the same time the insitu reinforcement concrete floor are designed in robust connection into the shear wall (vertical element) and performing the function as a horizontal diaphragms to transfer lateral loads to the vertical element and subsequently into the foundation. Please refer the below illustration of "Diaphragm of Shear wall (Figure B), 'Shear Wall Action (Figure C)', 'Diaphragm Action (Figure D)' and

For this 10 storey office building, it is design with a symmetrical floor layout and this layout make it suitably to create few shear wall system. Moreover this shear wall design and its implementation will only involve some minor arrangement and structural design alteration, such as;

  • alter the type of foundation,
  • convert the existing wall element become vertical reinforcement concrete wall,
  • change the floor system in order it must be robust enough and able to transfer the lateral force to the shear wall within the limit of design deflection.
  • Similarly the design of the beam (underneath at open space area) must be rigidly tied into the supporting shear walls make sure the lateral load can efficiently for transfer to the foundation.

With the above alteration the whole shear wall will be formed (refer layout plan grid line A-B/3-6 & F-G/3-6) and this new created system not only carries vertical load but it floor system acting as a diaphragm to transfer the lateral loads to this shear wall and then transfer those load to the foundation efficiently. With this alternative design the building height will able to achieve 30 storeys and maximum to achieve 35 storeys height. Besides, through this approach the overall building concept will be remained unchange.

B) Introduction of Frame Using Vertical Trusses, Belt Trusses and /or Outrigger Trusses

This concept is development from the earlier shear wall. The purpose of this design is to provide a vertical truss call 'Outrigger Trusses' where it is robustly connected to the perimeter column/exterior column with the purpose to form a much stiffer structure at external column and enables it to resist greater wind forces or other form of lateral load, as well as the resistant of gravity load. In addition the 'Belt Trusses' will be built and it is functioning in wrapping through the perimeter column at the same level as the outrigger trusses to further stiffen of the structure.

This design is suitable in apply to amend this building height into 10 storey. The Figure E and Figure F below shown two diagrams to illustrates the concept of this vertical trusses, belt trusses and outrigger trusses.

In order to apply this concept in this project, the perimeter building column (facade column) will be tied by using beam act as outrigger trusses and robustly connected to central core (also act as shear wall). Through this alternative frame solution the whole structure will act as a large system will enabling a structure approximately 25% stiffer than a original structure solely relying on a shear truss or shear wall system while without changing the floor plan arrangement. Figure F shown how the outrigger truss combine with shear wall to further enhance this whole building structure to resist lateral forces.

C) Introduction of Tube System and Bundle Tube System

The tube system have been the most efficient structural system used for tall building. This tube approach creates a 3 dimensional system (Refer Figure G). This system can be formed by using the conventional frame system where the external columns around the perimeter of the building are designed in much closer together. Subsequently, these columns around the corners of the four building facades are tied robustly with short beam and will form a continuity system around all four facades and effectively create a structure similar to a huge box section that cantilevers from the foundation to the top of the building. This structural system capable in resisting lateral forces in any direction as in principle a 'Box" section has inherent strength.

This tube system is appropriate to adopt in order to increase this building height to 30 storeys. From this building existing layout design (in term of shape) it is suitably to form a 'Tube shape'. Therefore, according to this tube system design criteria, the perimeter column (facade column) of this building will be placed much closer (Refer Figure H) and tied robustly by beam surrounded the four facades of the building and become more stiffened. This will enable the whole structure act as a whole system to resist the lateral force as well as functioning to transfer the gravity load to the foundation. The advantages of this tubes system is it is allow fewer interior columns, and so create more usable floor space.

In addition the above propose tube system can wisely interconnected or combine act as a whole with the existing tube frames, i.e. two number of lift core and the alternative proposed new shear walls (position at grid line A-B/3-6 & F-G/3-6), to create and perform another approach call 'bundle tube system'. Through this combination a stronger structure will be created and efficiently to resist the lateral forces and gravity loads. The Figure F illustrate the bundle tube system and through this integrate structural frame it is definitely will become more efficiently to resists lateral loads or "sway" force cause by wind.

As a conclusion, with the above solution of alternative frame design (i.e shear walls, vertical trusses, belt trusses and Outrigger Trusses, tube system). This building will be able to increase to 30 storey height with any one of the above single alternative frame design. However it is advisable to combine those approaches by looking at the advantages and permission of this existing symmetrical building layout with the objective to resists significant lateral loads or "sway" force (cause by wind), as well as cope efficiently with the gravity loads (vertical load). Moreover theses combination will not cause much changes in terms of the original design and the building concept.

References:

1 Chew, Y L M., Construction Technology for Tall Buildings (2nd Edition),

2 Construction Technology 5, Heriot-Watt University

3 Brick Industry Association, Technical Notes 24C-The Contemporary Bearing Wall, Introduction To Shear Wall Design, (Sept./Oct.1970)(Reissued May 1988) www.gobrick.com

4 Tall Building Structures Analysis And Design, By Bryan Stafford Smith, Alex Coull)

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